Heat exchangers of multiple cylinder internal combustion engine

ABSTRACT

First heat exchangers (H 4,  H 3,  H 2 ) are disposed within an exhaust port ( 18 ), within a pre-catalytic device ( 34 ), and on the downstream of a main catalytic device ( 35 ); the exhaust port ( 18 ), the pre-catalytic device ( 34 ), and the main catalytic device ( 35 ) being provided in each exhaust passage ( 33 ) of a multicylinder internal combustion engine. These first heat exchangers (H 4,  H 3,  H 2 ) are independently provided for each of the exhaust passages ( 33 ), and a second heat exchanger (H 1 ) is disposed in a section where these exhaust passages ( 33 ) are combined. Since the first heat exchangers (H 4,  H 3,  H 2 ) are disposed on the upstream side of the exhaust passage ( 33 ), high heat exchange efficiency can be obtained by high temperature exhaust gas, and, moreover, the occurrence of exhaust interference can be avoided, thereby preventing any decrease in the output of the internal combustion engine.

This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP01/00348 which has an International filing date of Jan. 19, 2001, which designated the United States of America.

FIELD OF THE INVENTION

The present invention relates to a heat exchanger for carrying out heat exchange between a heat medium and an exhaust gas from a multicylinder internal combustion engine.

BACKGROUND ART

An arrangement in which a heat exchanger of an evaporator, which heats water to make vapor using exhaust gas heat, is disposed in a combined section of an exhaust manifold of a multicylinder internal combustion engine is known from Japanese Patent Application Laid-open No. 5-340241. Furthermore, an arrangement in which a heat exchanger of an evaporator is disposed in each of the cylinders of a multicylinder internal combustion engine is known from Japanese Patent Application Laid-open No. 56-156407.

However, in the arrangement described in Japanese Patent Application Laid-open No. 5-340241, since the exhaust gas coming out of an exhaust port passes through the exhaust manifold and flows into the heat exchanger, the temperature of the exhaust gas decreases while passing through the exhaust manifold, and there is therefore the problem that the heat exchange efficiency of the heat exchanger deteriorates.

Furthermore, in the arrangement described in Japanese Patent Application Laid-open No. 56-156407, not only are as many evaporators required as there are cylinders, but also pulsations in the exhaust gas immediately after coming out of the exhaust port act on the heat exchanger, and there is therefore the problem that the heat exchange efficiency of the heat exchanger deteriorates at the moment at which the flow of the exhaust gas pulses and stops.

DISCLOSURE OF THE INVENTION

The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to enhance the performance of a heat exchanger that carries out heat exchange between a heat medium and an exhaust gas of a multicylinder internal combustion engine.

In order to accomplish the object, in accordance with the present invention there is proposed a heat exchanger for a multicylinder internal combustion engine, wherein a first heat exchanger is disposed in each exhaust passage extending from a corresponding combustion chamber of a multicylinder internal combustion engine, and a second heat exchanger is disposed in a section where at least two exhaust passages are combined.

In accordance with this arrangement, since the first heat exchanger is disposed in each exhaust passage extending from the corresponding combustion chamber of the multicylinder internal combustion engine, high temperature exhaust gas can be made to act on the first heat exchanger immediately after coming out of the combustion chamber, thus achieving high heat exchange efficiency, and, moreover, the occurrence of exhaust interference can be avoided, thereby preventing any decrease in the output of the internal combustion engine. Furthermore, since there is little pressure loss in the exhaust gas immediately after coming out of the combustion chamber, enhancement of the heat exchange efficiency due to a heat transfer promoting effect of the pulsed exhaust flow can be expected. Moreover, since the second heat exchanger is disposed in the section where at least two exhaust passages are combined, the exhaust gas having no pulsation and a uniform temperature as a result of combining the flows can be made to act on the second heat exchanger, thereby enhancing the heat exchange efficiency.

Furthermore, in addition to the above-mentioned arrangement, there is proposed a heat exchanger for a multicylinder internal combustion engine, wherein the first heat exchanger positioned on the upstream side of the flow of exhaust gas and the second heat exchanger positioned on the downstream side communicate with each other.

In accordance with this arrangement, since the first heat exchanger positioned on the upstream side and the second heat exchanger positioned on the downstream side communicate with each other, the exhaust gas passes through the first heat exchanger and the second heat exchanger sequentially and undergoes effective heat exchange.

Moreover, in addition to the above-mentioned arrangement, there is proposed a heat exchanger for a multicylinder internal combustion engine, wherein the first heat exchanger and the second heat exchanger are integrated.

In accordance with this arrangement, since the first heat exchanger and the second heat exchanger are integrated, not only does mounting the heat exchangers on the internal combustion engine and removing them become easy, thereby enhancing the ease of maintenance, but also the integrated structure increases the rigidity of the heat exchangers, thereby enhancing the durability against vibration of the internal combustion engine.

Furthermore, in addition to the above-mentioned arrangement, there is proposed a heat exchanger for a multicylinder internal combustion engine, wherein the second heat exchanger is disposed in a layer around the outer periphery of the first heat exchanger.

In accordance with this arrangement, since the second heat exchanger is disposed in a layer around the outer periphery of the first heat exchanger, the a fourth stage heat exchanger; FIG. 9B is a view from arrow b in FIG. 9A; FIG. 9C is a view from arrow c in FIG. 9A; FIG. 10 is an exploded perspective view of a pre-catalytic system; FIG. 11 is a schematic diagram showing a water supply route of an evaporator; FIG. 12 is an exploded perspective view of the evaporator; FIG. 13 is a diagram showing the layout of catalytic systems and heat exchangers in an embodiment and comparative embodiments; FIG. 14 is a graph showing the relationship between the exhaust gas temperature and the distance from an exhaust port when cold starting; FIG. 15 is a graph showing the relationship between the exhaust gas temperature and the distance from the exhaust port at high temperature; FIG. 16 is a graph explaining the effect of multiple water supplies; FIG. 17 is a graph showing the relationship between the Reynolds number and the heat transfer performance for steady flow and pulsed flow; and FIG. 18 is a graph showing the relationship between the Reynolds number and the heat transfer performance at different exhaust pressures.

BEST MODE FOR CARRYING OUT THE INVENTION

A mode for carrying out the present invention is explained below by reference to an embodiment of the present invention illustrated in the attached drawings.

In FIG. 1, a waste heat recovery system 2 for an internal combustion engine 1, as a combustion system mounted in an automobile, includes an evaporator 3 that generates vapor having increased temperature and pressure, that is, high-pressure vapor, using, as a heat source, waste heat such as, for example, exhaust gas from the internal combustion engine 1; an expander 4 that generates a shaft output by expansion of the high-pressure vapor; a condenser 5 that liquefies the vapor having decreased temperature and pressure, that is, low-pressure vapor, discharged from the expander 4 after the expansion; and a water supply pump 6 that supplies water from the condenser 5 to the evaporator 3.

As is clear by referring also to FIG. 2, a power transmission system 121 connected to the waste heat recovery system 2 includes a planetary gear mechanism 122, a belt type continuously variable transmission 123, and an electric generator/motor 124.

The planetary gear mechanism 122 includes a sun gear 125, a ring gear 126, a planetary carrier 127, and a plurality of planetary gears 128 axially supported by the planetary carrier 127 and meshing simultaneously with the sun gear 125 and the ring gear 126. The planetary carrier 127 connected to an output shaft 129 of the expander 4 can engage with a casing, which is not illustrated, via a carrier brake 130. The sun gear 125 connected to an input/output shaft 131 of the electric generator/motor 124 can engage with the casing, which is not illustrated, via a sun gear brake 132. The ring gear 126 can engage with the casing, which is not illustrated, via a ring gear brake 133. Each of the carrier brake 130, the sun gear brake 132, and the ring gear brake 133 is formed from a hydraulic brake or an electromagnetic brake.

The electric generator/motor 124 is connected to a battery 134 that can be charged and discharged. The electric generator/motor 124 charges the battery 134 when it is driven by the shaft output of the expander 4 or the internal combustion engine 1 so as to function as an electric generator, and it assists the drive by the internal combustion engine 1 of driven wheels or starts the internal combustion engine 1 when it functions as a motor powered by the battery 134.

The belt type continuously variable transmission 123 includes a drive pulley 136 provided on an input shaft 135, a follower pulley 138 provided on an output shaft 137, and an endless belt 139 wrapped around the two pulleys 136, 138. The groove width of the drive pulley 136 and the groove width of the follower pulley 138 are individually variable by hydraulic control or electric control; increasing the groove width of the drive pulley 136 and decreasing the groove width of the follower pulley 138 continuously change the gear ratio to the LOW side, and decreasing the groove width of the drive pulley 136 and increasing the groove width of the follower pulley 138 continuously changes the gear ratio to the TOP side.

A drive gear 140 provided on the ring gear 126 of the planetary gear mechanism 122 meshes with a driven gear 141 provided on the input shaft 135 of the belt type continuously variable transmission 123. The internal combustion engine 1 is transmitted to a transmission 143 via an output shaft 142, and the output from the transmission 143 is transmitted to driven wheels, which are not illustrated. A drive gear 144 provided on the output shaft 137 of the belt type continuously variable transmission 123 meshes with a driven gear 145 provided on the output shaft 142 of the internal combustion engine 1.

Torque limiters 146, 147 are provided on the output shaft 129 of the expander 4 and the input/output shaft 131 of the electric generator/motor 124 respectively. The torque limiters 146, 147 slip when a torque equal to or greater than a predetermined value is applied to the expander 4 or the electric generator/motor 124, thereby preventing an excess load being generated. The torque limiters 146, 147 can be replaced with clutches that disengage when an overload torque that is equal to or greater than a predetermined value is generated. A clutch 148 is provided on the output shaft 137 of the belt type continuously variable transmission 123. The clutch 148 is for preventing an overload from being applied to the expander 4 due to the driving force transmitted back from the internal combustion engine 1 or the driven wheels, and it provides a connection between the internal combustion engine 1 and the expander 4 when it is engaged, and disconnects the internal combustion engine 1 from the expander 4 when it is disengaged.

When the sun gear 125 is fixed by engaging the sun gear brake 132 of the planetary gear mechanism 122, each of the planetary carrier 127 and the ring gear 126 becomes an input element or an output element; a driving force input from the expander 4 into the planetary carrier 127 is output to the ring gear 126 and is then transmitted therefrom to the output shaft 142 of the internal combustion engine 1 via the drive gear 140, l the driven gear 141, the belt type continuously variable transmission 123, the drive gear 144, and the driven gear 145, and the shaft output of the expander 4 can thereby assist the shaft output of the internal combustion engine 1. On the other hand, if a driving force is transmitted via the reverse of the above route when starting the expander 4, the shaft output of the internal combustion engine 1 can smoothly start the expander 4.

When the ring gear 126 is fixed by engaging the ring gear brake 133 of the planetary gear mechanism 122, each of the expander 4 or the electric generator/motor 124 becomes an input element and an output element; a driving force input from the expander 4 into the planetary carrier 127 is output to the electric generator/motor 124 via the sun gear 125, thus allowing the electric generator/motor 124 to function as an electric generator, and thereby charging the battery 134. On the other hand, if a driving force is transmitted via the reverse of the above route when starting the expander 4, the shaft output of the electric generator/motor 124 functioning as a motor can smoothly start the expander 4.

When the planetary carrier 127 is fixed by engaging the carrier brake 130 of the planetary gear mechanism 122, each of the sun gear 125 and the ring gear 126 becomes an input element or an output element. A driving force input into the sun gear 125 from the electric generator/motor 124 functioning as a motor is therefore output from the ring gear 126, is transmitted therefrom to the output shaft 142 of the internal combustion engine 1 via the drive gear 140, the driven gear 141, the belt type continuously variable transmission 123, the drive gear 144, and the driven gear 145, and assists the shaft output of the internal combustion engine 1 or starts the internal combustion engine 1. On the other hand, transmitting the shaft output of the internal combustion engine 1 to the electric generator/motor 124 via the reverse of the above route allows the electric generator/motor 124 to function as an electric generator, thereby charging the battery 134.

The structure of the evaporator 3 of the waste heat recovery system 2 for the internal combustion engine 1 is now explained in detail by reference to FIGS. 3 to 12.

As shown in FIGS. 3 to 8, the in-line three cylinder internal combustion engine 1 includes a cylinder block 11, a cylinder head 12, and a head cover 13, which are laminated one on another, and pistons 15 are slidably fitted in three cylinder bores 14 formed in the cylinder block 11. Among intake ports 17 and exhaust ports 18 communicating with three corresponding combustion chambers 16 formed in the cylinder head 12, the intake ports 17 are bored within the cylinder head 12 as is conventional, but the exhaust ports 18 are formed from a separate member and joined to the cylinder head 12.

The upper end of a stem 21 of an intake valve 20 that opens and closes an intake valve hole 19 abuts against one end of an intake rocker arm 23 pivotably supported on an intake rocker arm shaft 22, and the upper end of a stem 26 of an exhaust valve 25 that opens and closes an exhaust valve hole 24 abuts against one end of an exhaust rocker arm 28 pivotably supported on an exhaust rocker arm shaft 27. The other end of the intake rocker arm 23 and the other end of the exhaust rocker arm 28 abut against an intake cam 30 and an exhaust cam 31 respectively provided on a camshaft 29 rotating in association with a crankshaft, which is not illustrated, thereby making the intake valve 20 and the exhaust valve 25 open and close.

Provided on the side face of the cylinder head 12 on the exhaust side is the evaporator 3 that generates vapor having increased temperature and pressure, that is, high-pressure vapor, using the exhaust gas of the internal combustion engine 1 as a heat source. The evaporator 3 includes an exhaust passage 33 having the three exhaust ports 18 as the base end and extending to an exhaust pipe 32, three pre-catalytic systems 34 and three main catalytic systems 35 disposed within the exhaust passage 33, and heat exchangers H1 to H5 carrying out heat exchange with the exhaust gas flowing in the exhaust passage 33.

Each of the exhaust ports 18 is formed from a uniform diameter part 18 a positioned on the upstream side of the flow of the exhaust gas, and having a substantially constant diameter, and an increasing diameter part 18 b provided so as to be connected to the downstream side of the uniform diameter part 18 a and having a diameter that increases in a trumpet shape; the fifth stage heat exchanger H5 is provided around the outer periphery of the uniform diameter part 18 a, and the fourth stage heat exchanger H4 is provided within the increasing diameter part 18 b. The fifth stage heat exchanger H5 is formed from about 5 turns of a single heat transfer tube 37 wound around the outer periphery of the uniform diameter part 18 a. The fourth stage heat exchanger H4 is formed from multiple windings of a single heat transfer tube 38 that is housed within the increasing diameter part 18 b, and the heat transfer tube 37 of the fifth stage heat exchanger H5 runs through an opening (not illustrated) formed in the exhaust port 18 and is continuous to the heat transfer tube 38 of the fourth stage heat exchanger H4.

As is clear from reference to FIGS. 9A to 9C, the heat transfer tube 38 of the fourth stage heat exchanger H4 is wound in a triple coil shape that is tapered so as to follow the shape of the interior of the increasing diameter part 18 b of the exhaust port 18; the coil in the inner layer is wound from the rear (the left-hand side in the figure) toward the front (the right-hand side in the figure) while decreasing in diameter and is folded back at the front end; this is followed by the coil in the middle layer, which is wound from the front toward the rear while increasing in diameter and is folded back at the rear end; and this is followed by the coil in the outer layer, which is wound from the rear toward the front while decreasing in diameter. A water inlet shown in FIG. 9B is connected to the third stage heat exchanger H3, which is on the upstream side and will be described later, and a water outlet shown in FIG. 9C is connected to the heat transfer tube 37 of the fifth stage heat exchanger H5, which is on the downstream side. The circled numerals {circle around (1)} to {circle around (6)} shown in FIG. 9A show the route via which water flows through the heat transfer tube 38.

In addition, winding the heat transfer tube 38 of the fourth stage heat exchanger H4 in the triple coil shape that is tapered so as to follow the shape of the interior of the increasing diameter part 18b of the exhaust port 18 makes it possible to have a rectifying effect on the exhaust gas that flows through the increasing diameter part 18 b, thereby contributing to a reduction in the circulation resistance.

As is most clearly shown in FIGS. 7 and 8, an annular distribution passage forming member 41 is integrally formed on the rear end of the increasing diameter part 18 b of the exhaust port 18, and by joining a separate annular distribution passage forming member 42 to the rear face of the distribution passage forming member 41, a third circular distribution passage 43 is formed between the two distribution passage forming members 41, 42. The upstream end of the heat transfer tube 38 of the fourth stage heat exchanger H4 is connected to the third circular distribution passage 43.

The front end of a cylindrical case 44 covering the outer periphery of the pre-catalytic system 34 is joined to the distribution passage forming member 42, and a second circular distribution passage 47 is formed between two annular distribution passage forming members 45, 46, which are superimposed one on another and joined to the rear end of the cylindrical case 44. The pre-catalytic system 34 and the third stage heat exchanger H3 are disposed within the cylindrical case 44.

The pre-catalytic system 34 includes seven sheets of catalyst support 48 formed in honeycomb plates, on the surface of which is supported a known exhaust gas purification catalyst. The third stage heat exchanger H3, which is disposed within the cylindrical case 44 so as to surround the seven sheets of catalyst support 48, is formed from two bent heat transfer tubes 49, 49 (see FIG. 10). Each of the heat transfer tubes 49, 49 is bent in a zigzag within a circular plane, then moves to the next plane that is separated therefrom by one pitch in the axial direction and is bent in the same zigzag shape, this being repeated to give a cylindrical outer shape having a plurality of pitches. The seven sheets of catalyst support 48 are housed within the internal space formed by interlacing together the two heat transfer tubes 49, 49. Here, the two heat transfer tubes 49, 49 are in intimate contact with the surface of the seven sheets of catalyst support 48. The upstream ends of the two heat transfer tubes 49, 49 are connected to the second circular distribution passage 47 formed between the distribution passage forming members 45, 46, and the downstream ends thereof are connected to the third circular distribution passage 43 formed between the distribution passage forming members 41, 42.

Two cylindrical cases 50, 51 are coaxially disposed outside, in the radial direction of the cylindrical case 44 of the pre-catalytic system 34, and the second stage heat exchanger H2 is disposed in an annular form between the two cylindrical cases 50, 51. The second stage heat exchanger H2 is formed from a large number of heat transfer tubes 52 wound in a coiled shape in one direction and a large number of heat transfer tubes 53 wound in a coiled shape in the other direction, the tubes 52, 53 being disposed alternately so that parts thereof are meshed together, thereby increasing the placement density of the heat transfer tubes 52, 53 within the space. The outer periphery of the pre-catalytic system 34 is thus surrounded by the heat transfer tubes 52, 53.

A first circular distribution passage 56 is formed between a distribution passage forming member 54 fixed to the front end of the cylindrical case 50 on the outer side and a distribution passage forming member 55 joined to the front face of the distribution passage forming member 54. The upstream ends of the heat transfer tubes 52, 53 are connected to the first circular distribution passage 56, and the downstream ends of the heat transfer tubes 52, 53 are connected to the second circular distribution passage 47.

The three pre-catalytic systems 34 are combined into one by a press-formed metal plate mounting plate 57 and fixed to the cylinder head 12. Three openings 57a are formed in the mounting plate 57, and the distribution passage forming member 41 of each of the increasing diameter parts 18 b of the three exhaust ports 18 is integrally fixed to the corresponding opening 57 a. An oval-shaped flange 58 fixed to the outer periphery of the mounting plate 57 is fixed to the cylinder head 12 by sixteen bolts 59.

The three main catalytic systems 35 are disposed to the rear of the three pre-catalytic systems 34. The main catalytic systems 35 are formed by supporting a catalyst on the surface of catalyst supports 60 having a honeycomb structure formed in an overall cylindrical shape, and thick ring members 61 are fitted around the outer peripheries thereof. The main catalytic systems 35 have a diameter larger than that of the pre-catalytic systems 34, and the main catalytic systems 35 are divided into inner layer parts 35 a having the same diameter as that of the pre-catalytic systems 34 and outer layer parts 35 b that project outside the outer peripheries of the pre-catalytic systems 34. In order to seal opposing parts of the pre-catalytic systems 34 and the main catalytic systems 35, seal members 63 supported on the rear face of the distribution passage forming member 46 via springs 62 resiliently abut against the front faces of the main catalytic systems 35. End caps 65 are supported, via springs 64, on the rear ends of the ring members 61 on the outer peripheries of the main catalytic systems 35. The rear faces of the three end caps 65 abut against projections 66 a provided on the front face of an inner wall member 66, which will be described later, and are pushed forward.

The outsides of the three pre-catalytic systems 34 and the three main catalytic systems 35 are covered with a detachable common cover 71. The cover 71 includes a plate-shaped distribution passage forming member 72 having a mounting hole 72 a for the exhaust pipe 32 in its center and a triple ring-shaped distribution passage forming member 73 joined to the front face of the distribution passage forming member 72, and a first triple ring-shaped distribution passage 74 is formed between the two distribution passage forming members 72, 73. A tubular member 75 positioned radially outside and a tubular member 76 positioned radially inside extend forward, with a slight gap therebetween, from the triple ring-shaped distribution passage forming member 73, and an oval flange 77 provided on the front end of the outer tubular member 75 is superimposed on the flange 58 and they are tightened together by the bolts 59.

A triple ring-shaped distribution passage forming member 78 is fixed to the front end of the inner tubular member 76, and a second triple ring-shaped distribution passage 80 is formed by joining, to the front face of the distribution passage forming member 78, a distribution passage forming member 79 of the substantially same shape. The first triple ring-shaped distribution passage 74 and the second triple ring-shaped distribution passage 80 have an identical shape and face each other in the front to rear direction. The cup-shaped inner wall member 66 is housed within the cover 71, and the first stage heat exchanger H1 is disposed between the outer periphery of the inner wall member 66 and the inner periphery of the inner tubular member 76.

The first stage heat exchanger H1 has a similar structure to that of the second stage heat exchangers H2; a large number of heat transfer tubes 81 wound in a coiled shape in one direction and a large number of heat transfer tubes 82 wound in a coiled shape in the other direction are disposed alternately so that parts thereof are meshed together, and these heat transfer tubes 81, 82 surround the outer peripheries of the second stage heat exchangers H2 and the outer peripheries of the main catalytic systems 35. The upstream ends of the heat transfer tubes 81, 82 are connected to the first triple ring-shaped distribution passage 74, and the downstream ends thereof are connected to the second triple ring-shaped distribution passage 80.

The materials for the heat transfer tubes 37 of the fifth stage heat exchangers H5, the heat transfer tubes 38 of the fourth stage heat exchangers H4, the heat transfer tubes 49 of the third stage heat exchangers H3, the heat transfer tubes 52, 53 of the second stage heat exchangers H2, and the heat transfer tubes 81, 82 of the first stage heat exchanger H1 are preferably heat-resistant stainless steel (austenite type such as SUS 316L or SUS 310S, ferrite type such as SUS 430 or SUS 444) or a nickel-based heat-resistant alloy. Joining of the heat transfer tubes is preferably carried out by brazing or mechanical restraint.

Furthermore, with regard to the catalyst supports 48 for the pre-catalytic systems 34, heat-resistant stainless steel (e.g., 20% by weight Cr—5% by weight Al ferrite type stainless steel) or a nickel-based heat-resistant alloy foil (thickness 0.1 mm or below) is preferable, and with regard to the catalyst supports 60 for the main catalytic systems 35, cordylite is preferable.

As is clear from reference to FIG. 11, a water inlet 83, into which water that is a source of high pressure vapor is supplied, is provided in a central part of the first triple ring-shaped distribution passage 74, which communicates with the second triple ring-shaped distribution passage 80 via a large number of the heat transfer tubes 81, 82 of the first stage heat exchanger H1 disposed so as to surround the outer peripheries of the three main catalytic systems 35, and the second triple ring-shaped distribution passage 80 communicates with the three first circular distribution passages 56 via two detachable couplings 84.

The three first circular distribution passages 56 communicate with the three second circular distribution passages 47 via the heat transfer tubes 52, 53 of the second stage heat exchangers H2 disposed so as to surround the outer peripheries of the three pre-catalytic systems 34, and each of these three second circular distribution passages 47 communicates with the corresponding one of the three third circular distribution passages 43 via two of the heat transfer tubes 49 of the third stage heat exchangers H3 disposed within the three pre-catalytic systems 34. Each of the three third circular distribution passages 43 continues through one of the heat transfer tubes 38 of the fourth stage heat exchangers H4 that pass through the interiors of the three exhaust ports 18 and one of the heat transfer tubes 37 of the fifth stage heat exchangers H5 that pass around the exteriors of the three exhaust ports 18, and they are then combined together by a coupling 85 and supplied to the expander 4 in a subsequent stage from a water outlet 86.

Water that has been supplied from a midstream water inlet 87 branches in three directions in a distributor 88, a part thereof is supplied midstream to the three first circular distribution passages 56 via the couplings 84, a part thereof is supplied midstream to the three second circular distribution passages 47, and a part thereof is supplied midstream to the three third circular distribution passages 43.

In this way, while the water that is supplied from the water inlet 83 travels to the water outlet 86 via the first stage heat exchanger H1→the second stage heat exchangers H2→the third stage heat exchangers H3→the fourth stage heat exchangers H4→the fifth stage heat exchangers H5, it exchanges heat with exhaust gas that comes out of the internal combustion engine 1 and flows in a direction opposite to that in which the water flows, the water becoming vapor.

That is, while passing through the uniform diameter parts 18 a of the three exhaust ports 18, the exhaust gas coming out of the internal combustion engine 1 exchanges heat with the fifth stage heat exchangers H5 formed from the heat transfer tubes 37 wound around the outer peripheries of the uniform diameter parts 18 a. The exhaust gas that has flowed from the uniform diameter parts 18 a of the exhaust ports 18 into the increasing diameter parts 18 b exchanges heat by direct contact with the fourth stage heat exchangers H4 formed from the heat transfer tubes 38 wound in a triple coil shape and housed within the increasing diameter parts 18 b. The exhaust gas passes from the exhaust ports 18 through the interior of the seven catalyst supports 48 of each of the three pre-catalytic systems 34 to purify its harmful components and, at this point, exchanges heat with the third stage heat exchangers H3 formed from the heat transfer tubes 49 surrounding the peripheries of the catalyst supports 48.

The exhaust gas that has passed through the three pre-catalytic systems 34 passes through the inner layer parts 35 a of the three main catalytic systems 35 from the front to the rear, is then blocked by the end caps 65 and makes a U-turn, and passes through the outer layer parts 35 b of the main catalytic systems 35 from the rear to the front; during this stage, harmful components in the exhaust gas are purified by the main catalytic systems 35. The exhaust gas coming out of the main catalytic systems 35 exchanges heat while flowing, from the rear to the front, through the second stage heat exchangers H2 formed from the heat transfer tubes 52, 53 disposed between the pairs of cylindrical cases 50, 51, then changes direction through 180°, exchanges heat while flowing, from the front to the rear, through the first stage heat exchanger H1 formed from the heat transfer tubes 81, 82 disposed between the tubular member 76 and the inner wall member 66, and is finally discharged into the exhaust pipe 32 through the mounting hole 72 a of the distribution passage forming member 72.

The procedure for assembling the evaporator 3 having the above-mentioned structure is explained mainly by reference to FIG. 12.

Firstly, mounted in the cylinder head 12 is a sub-assembly integrally formed from the three exhaust ports 18, to which the fourth stage heat exchangers H4 and the fifth stage heat exchangers H5 are preassembled, and the three pre-catalytic systems 34, to which the second stage heat exchangers H2 and the third stage heat exchangers H3 are preassembled. That is, the distribution passage forming members 41 provided in the three exhaust ports 18 are integrally fixed to the three openings 57 a of the plate-shaped mounting plate 57, and the oval-shaped flange 58 fixed to the outer periphery of the mounting plate 57 is positioned on the cylinder head 12.

Subsequently, the three main catalytic systems 35 are brought up to the three pre-catalytic systems 34 from the rear, and the outer peripheries at the front end of the ring members 61 on the outer peripheries of the main catalytic systems 35 are fitted to the outer peripheries at the rear end of the cylindrical cases 50 of the second stage heat exchangers H2. At this point, the seal members 63 supported on the rear faces of the distribution passage forming members 46 via the springs 62 resiliently abut against the front faces of the main catalytic systems 35 (see FIG. 4).

Subsequently, the cover 71 is moved forward so as to cover the outer peripheries of the three main catalytic systems 35 and the three second stage heat exchangers H2 with the triple ring-shaped first stage heat exchanger H1 in which three circles are staggered in the lateral direction and superimposed one on another, and the flange 77 provided on the tubular member 75 of the cover 71 is superimposed on the rear face of the flange 58 of the mounting plate 57 and joined to the cylinder head 12 by the sixteen bolts 59. At this point, the projections 66 a of the inner wall member 66 within the cover 71 push the end caps 65 of the main catalytic systems 35 forward so as to compress the springs 64 between spring seats 67 provided on the outer peripheries of the end caps 65 and spring seats 68 provided on the rear ends of the ring members 61 on the outer peripheries of the main catalytic systems 35 (see FIG. 4).

As hereinbefore described, since the assembly is carried out so as to give a slight gap in the radial direction between an inner layer part including the pre-catalytic systems 34 and the main catalytic systems 35, and the cover 71, which is an outer layer part covering the outer peripheries thereof, thermal expansion thereof in the radial direction can be absorbed. Furthermore, since the main catalytic systems 35 are resiliently retained by the springs 62 and 64 between the rear faces of the pre-catalytic systems 34 and the front face of the inner wall member 66 of the cover 71, thermal expansion of the pre-catalytic systems 34 and the main catalytic systems 35 in the axial direction can be absorbed.

Finally, the three first circular distribution passages 56 are each connected to the second triple ring-shaped distribution passage 80 at the front end of the cover 71 via the couplings 84, and the three heat transfer tubes 37 of the fifth stage heat exchangers H5 extending from the three exhaust ports 18 are combined by the coupling 85 to thereby complete the assembly of the evaporator 3.

It should be noted here that as shown in FIG. 13, in Present Embodiment P-1, a catalytic system for purifying an exhaust gas is divided into the pre-catalytic systems 34 on the upstream side and the main catalytic systems 35 on the downstream side, the fourth stage heat exchangers H4 and the fifth stage heat exchangers H5 are disposed on the upstream side of the flow of the exhaust gas, of the pre-catalytic systems 34, and the first stage heat exchanger H1 and the second stage heat exchangers H2 are disposed on the downstream side of the flow of the exhaust gas, of the main catalytic systems 35. As hereinbefore described, the third stage heat exchangers H3 are housed within the pre-catalytic systems 34.

On the other hand, Comparative Example C-0 includes neither a catalytic system nor a heat exchanger, Comparative Example C-1 includes only a main catalytic system, Comparative Example C-2 includes a main catalytic system as the stage following a pre-catalytic system, and Comparative Example C-3 includes heat exchangers as both the stage prior to and the stage following a main catalytic system.

FIG. 14 shows the change in temperature of the exhaust gas from the upstream side to the downstream side (L0→L1→L2→L3→L4→L5) during a cool period immediately after starting the internal combustion engine 1. In accordance with Present Embodiment P-1, since the pre-catalytic systems 34 are disposed on the upstream side of the exhaust passage 33 and the capacity of the pre-catalytic systems 34 is set small, the temperature of the pre-catalytic systems 34 can be increased quickly to the catalyst activation temperature or above even during the cool period, thereby reducing the harmful components in the exhaust gas effectively.

Moreover, although the exhaust gas purification performance would be insufficient with only the small capacity pre-catalytic systems 34, disposition of the main catalytic systems 35 having a large capacity on the downstream side thereof can adequately compensate for the exhaust gas purification performance of the small capacity pre-catalytic systems 34. Furthermore, since the direction of flow of the exhaust gas is inverted between the inner layer parts 35 a and the outer layer parts 35 b of the main catalytic systems 35, when the exhaust gas passes firstly through the inner layer parts 35 a of the main catalytic systems 35, its temperature increases due to the catalytic reaction, the exhaust gas having increased temperature is supplied to the outer layer parts 35 b of the main catalytic systems 35, and when the exhaust gas turns around through 180°, harmful components in the exhaust gas are mixed effectively thus promoting the catalytic reaction in the outer layer parts 35 b, and thereby enhancing the overall exhaust gas purification performance of the main catalytic systems 35. Moreover, the exhaust gas in the inner layer parts 35 a is covered by the exhaust gas in the outer layer parts 35 b, thus preventing thermal leakage and thereby preventing any degradation in the heat exchange efficiency.

FIG. 15 shows the change in temperature of the exhaust gas from the upstream side to the downstream side when the internal combustion engine 1 is being operated at high temperature. In accordance with the Present Embodiment P-1, although there is a possibility of degradation due to exposure to high temperature exhaust gas because the pre-catalytic systems 34 are positioned close to the exhaust ports 18, since the fourth stage heat exchangers H4 and the fifth stage heat exchangers H5 are placed in the exhaust ports 18, it is possible to prevent the catalyst temperature of the pre-catalytic systems 34 from exceeding the heat resistant temperature. Furthermore, since, within the pre-catalytic systems 34, the catalyst supports 48 of the third stage heat exchangers H3 are divided into seven narrow pieces, and the zigzag-bent heat transfer tubes 49 of the third stage heat exchangers H3 are in direct contact with the peripheries of the catalyst supports 48, the catalyst temperature of the pre-catalytic systems 34 can more reliably be prevented from exceeding the heat resistant temperature.

Moreover, since the first stage heat exchanger H1, the second stage heat exchangers H2, the third stage heat exchangers H3, the fourth stage heat exchangers H4, and the fifth stage heat exchangers H5 are connected in line, and water is supplied sequentially from the first stage heat exchanger H1 side to the fifth stage heat exchangers H5 side (this water supply method is called one-way water supply), increasing/decreasing the amount of water supplied can appropriately control the temperatures of the pre-catalytic systems 34 and the main catalytic systems 35 according to the operational state of the internal combustion engine 1 (see Table 1).

TABLE 1 Comparison of cold starting characteristics and high temperature heat resistance High temperature Cold starting heat characteristics resistance (early (catalyst Overall activation) degradation) evaluation Com- C-1 Poor Poor Poor parative C-2 Good Poor Poor Exam- C-3 Poor Good Poor ple :Control of flow :No :No rate of low temperature medium in heat exchanger Em- P-1 Good Good Good bodi- :Control of flow :Yes :Yes ment rate of low (flow rate: low) (flow rate: temperature medium high) in heat exchanger

Furthermore, supplying water midstream at three positions in the water supply route from the first stage heat exchanger H1 to the fifth stage heat exchangers H5, that is, the first circular distribution passages 56 immediately upstream of the second stage heat exchangers H2, the second circular distribution passages 47 immediately upstream of the third stage heat exchangers H3, and the third circular distribution passages 43 immediately upstream of the fourth stage heat exchangers H4, and individually controlling the amount of water supplied to the second stage to fourth stage heat exchangers H2, H3, H4 according to changes in the operational state (the flow rate of the exhaust gas or the temperature of the exhaust gas) of the internal combustion engine 1 and the catalyst temperature (this water supply method is called multiple water supply) can yet more finely control the catalyst temperature of the pre-catalytic systems 34 and the main catalytic systems 35 at temperatures appropriate for the catalytic reaction (see Table 2 and FIG. 16).

TABLE 2 Ability to track catalyst temperature when thermal load fluctuates Operational status of combustion system (thermal load) Low thermal load Medium thermal load High thermal load Purific- Purific- Purific- ation ation ation Flow rate Temp. Track- perform- Temp. Track- perform- Temp. Track- perform- control method State of catalyst range ability ance range ability ance range ability ance One way water Pre-catalyst Good Rapid Good Good Rapid Good Good Rapid Good supply Main catalyst Lower Slow Poor Lower Slow Poor Lower Slow Poor Midstream (None) (None) (None) water :Total amount of :Small :Medium :Large supply water supplied Multiple water Pre-catalyst Good Rapid Good Good Rapid Good Good Rapid Good supply Main catalyst Good Rapid Good Good Rapid Good Good Rapid Good Midstream (Position of (A) (B) (C) (A) (B) (C) (A) (B) (C) water midstream supply water supply) :Midstream water :Trace :Trace :Trace :Trace :Low :Low :Low :Medium :High supplied :total amount of :Small :Medium :Large water supplied

The effect of multiple water supply is further explained by reference to FIG. 16. When carrying out one way water supply, as shown by a broken line, if the amount of water supplied is set small in line with a low load state of the internal combustion engine 1, the catalyst temperature passes the lower limit temperature (catalyst activation temperature) earlier, but it also quickly goes beyond the upper limit temperature (catalyst heat resistant temperature). Conversely, if the amount of water supply is set large in line with a high load state of the internal combustion engine 1, although the catalyst temperature passes the lower limit temperature (catalyst activation temperature) slowly, going beyond the upper limit temperature (catalyst heat resistant temperature) can be delayed. It is thus difficult to achieve both early activation and durability of the catalyst with the one way water supply, but by setting the amount of water supplied low when the internal combustion engine 1 is in a low load state and increasing the amount of water supplied by midstream water supply while the load is increasing, both early activation and durability of the catalyst can be achieved.

The reason why the lines for the pre-catalytic systems 34 are on the left side and the lines for the main catalytic systems 35 are on the right side in FIG. 16 is that the capacity of the pre-catalytic systems 34 is small and the capacity of the main catalytic systems 35 is large. It is of course possible to control the temperatures of the pre-catalytic systems 34 and the main catalytic systems 35 yet more finely by individually controlling the amounts of water supplied to the midstream water inlets at the three positions.

As hereinbefore described, in accordance with Present Embodiment P-1, in comparison with Comparative Examples C-0 to C-3, the overall exhaust gas purification performance and durability of the catalytic system can be enhanced. In particular, integral provision of the third stage heat exchangers H3 within the pre-catalytic systems 34 can actively control the temperature of the pre-catalytic systems 34, and midstream water supply to the first circular distribution passages 56, the second circular distribution passages 47, and the third circular distribution passages 43 in the vicinity of the pre-catalytic systems 34 can not only control the temperature of the pre-catalytic systems 34 themselves but can also appropriately control the temperature of the main catalytic systems 35 positioned downstream thereof, thereby greatly enhancing the overall exhaust gas purification performance.

With regard to the heat transfer surface densities (heat transfer area/volume) of the five heat exchangers H1 to H5, that of the first stage heat exchanger H1 is the highest, and the surface density gradually decreases therefrom toward the fifth stage heat exchangers H5. Furthermore, with regard to the passage cross sectional areas of the five heat exchangers H1 to H5, that of the first stage heat exchanger H1 is the smallest, and the cross sectional area gradually increases therefrom toward the fifth stage heat exchangers H5. The heat transfer surface densities and the passage cross sectional areas of the first stage to fourth stage heat exchangers H1 to H4 are shown in Table 3.

TABLE 3 Comparison of heat transfer surface density and passage cross sectional area among heat exchangers of different stages Heat transfer surface Passage cross density (m⁻¹) sectional area (m²) First stage heat exchanger 680 0.0008 Second stage heat exchanger 480 0.0009 Third stage heat exchanger 440 0.0009 Fourth stage heat exchanger 90 0.001

Gradually decreasing the heat transfer surface density (heat transfer area/volume) from the first stage heat exchanger H1 to the fifth stage heat exchangers H5 minimizes the heat transfer surface density of the fifth stage heat exchangers H5, through which high temperature exhaust gas passes because they are close to the combustion chambers 16, and maximizes the heat transfer surface density of the first stage heat exchanger H1, through which the exhaust gas whose temperature has decreased passes after passing through the exhaust passage 33, thereby averaging the heat exchange efficiencies across all of the five heat exchangers H1 to H5.

Furthermore, since the exhaust gas coming out of the combustion chambers 16 has a high temperature and a large volume, and as a result a high flow rate, maximizing the passage cross sectional area of the fifth stage heat exchangers H5 close to the combustion chambers 16 can minimize the pressure loss. On the other hand, since the exhaust gas that has decreased in temperature after passing through the exhaust passage 33 has decreased volume and also a decreased flow rate, minimizing the passage cross sectional area of the first stage heat exchanger H1 can make the evaporator 3 compact.

The effects obtained by setting the heat transfer surface densities and the passage cross sectional areas of the first stage heat exchanger H1 to the fourth stage heat exchangers H4 as shown in Table 3 are summarized in Table 4.

TABLE 4 Prior art (uniform structure)

It should be noted here that the second stage to fifth stage heat exchangers H2 to H5, which are heat exchangers in the earlier stage as seen from the internal combustion engine 1, are provided for each one of the exhaust ports 18, and since the exhaust gases coming from the exhaust ports 18 are not mixed, it is possible to avoid the occurrence of exhaust interference, thereby preventing any decrease in the output of the internal combustion engine 1. Furthermore, since there are pressure pulsations in the exhaust gas at the exits of the exhaust ports 18, and the exhaust pressure is high, a heat transfer promoting effect can be expected. FIG. 17 shows a comparison of the heat transfer performance at several Reynolds numbers between a hot air device without exhaust pulsations and an internal combustion engine with exhaust pulsations, and it confirms that the internal combustion engine with exhaust pulsations has the higher heat transfer performance. FIG. 18 shows a comparison of the heat transfer performance at several Reynolds numbers at two different exhaust pressures in a single cylinder internal combustion engine provided with a grouped pipe type heat exchanger, and it confirms that the higher the exhaust pressure, the higher the heat transfer performance.

In the first stage heat exchanger H1, which is a heat exchanger in the later stage as seen from the internal combustion engine 1, since the exhaust gases coming from the three exhaust ports 18 are combined into a non-pulsed flow, the exhaust gas can be maintained at a constant high temperature and, unlike pulsed flow, the exhaust gas can be made to have a steady flow that does not stop, thereby preventing any deterioration in the heat exchange performance.

Moreover, since the exhaust gas flows from the internal combustion engine 1 side to the exhaust pipe 32 side, whereas water flows from the exhaust pipe 32 side to the internal combustion engine 1 side, the exhaust gas and the water are in a cross-flow state, and the difference in temperature between the exhaust gas and the water can therefore be maximized across all of the first stage to fifth stage heat exchangers H1 to H5, thereby contributing to an enhancement of the heat exchange efficiency between the exhaust gas and the water.

Furthermore, as is clear from FIG. 4, the width of the evaporator 3 (width of the internal combustion engine 1 in the direction of the crankshaft) differs little from the width of the three cylinder bores 14, and it is extremely compact. Moreover, not only can the evaporator 3 be detached from the cylinder head 12 by merely loosening the sixteen bolts 59, thus providing ease of maintenance, but also the entire evaporator 3 is integrated with high rigidity by the cover 71, thereby enhancing the durability against vibration of the internal combustion engine 1.

Furthermore, since the exhaust passage 33 is bent into a three stage zigzag shape and the first stage to fourth stage heat exchangers H1 to H4 are disposed in layers in the radial direction, the overall dimensions of the evaporator 3 can be reduced as much as possible while minimizing thermal leakage and preventing noise from being dissipated from the interior of the evaporator 3, thereby providing a compact layout thereof in the cylinder head 12 of the internal combustion engine E.

Moreover, since the first stage to fifth stage heat exchangers H1 to H5 are arranged in a labyrinth form by disposing the pre-catalytic systems 34 and the main catalytic systems 35 in layers in the radial direction, not only can their silencing effect be effective in preventing exhaust noise from leaking outside the waste heat recovery system 2, but also an exhaust gas temperature lowering effect can be given, mainly by the first stage to fifth stage heat exchangers H1 to H5. This allows an exhaust muffler to be simplified or omitted, thereby making the exhaust system itself compact and lightweight. Furthermore, since the decrease in exhaust gas temperature causes the temperature of the exhaust passage to decrease, in particular, on the downstream side of the first stage heat exchanger H1, the degrees of freedom in designing with regard to heat resistance increase, and the use of a material such as a plastic for the exhaust passage becomes possible. As a result, the degrees of freedom in the shape of the exhaust passage, the degrees of freedom in mounting on a vehicle, the degrees of freedom in terms of cooling characteristics, etc. increase, and the degrees of freedom in the design of the entire vehicle, which has been subjected to restrictions by conventional exhaust systems, can be increased, thereby contributing to a reduction in the overall weight of the exhaust system.

Although an embodiment of the present invention is explained in detail above, the present invention can be modified in a variety of ways without departing from the spirit and scope thereof.

For example, in the embodiment, the first heat exchanger is formed from three heat exchangers, which are the second stage heat exchanger H2, the third stage heat exchanger H3, and the fourth stage heat exchanger H4, but the number of heat exchangers from which it is formed can be a number other than three. Similarly, in the embodiment, the second heat exchanger is formed from the first stage heat exchanger H1 alone, but the number of heat exchanges from which it is formed can be a number other than two.

Furthermore, in the embodiment, a three-cylinder internal combustion engine 1 is illustrated, but the present invention can also be applied to a multicylinder internal combustion engine having a number of cylinders other than three.

INDUSTRIAL APPLICABILITY

As hereinbefore described, the heat exchanger of a multicylinder internal combustion engine related to the present invention can be applied suitably to an internal combustion engine for an automobile, but it can also be applied to an internal combustion engine for any non-automobile application. 

1. A heat exchanger for a multicylinder internal combustion engine having a plurality of cylinders and exhaust passages extending from respective cylinders, comprising: a first heat exchanger (H2 to H4) disposed inside each of said exhaust passages (33) extending from a corresponding combustion chamber (16) of the multicylinder internal combustion engine (1) toward an exhaust pipe (32) at a location downstream from an exhaust valve (25), and a second heat exchanger (H1) disposed inside a section where at least two of the exhaust passages (33) extending from respective combustion chambers are combined such that said at least two exhaust passages communicate with said section and exhaust gases from the respective combustion chambers flow through said at least two exhaust passages, respectively, and then through said section, wherein at said section, the exhaust gases are combined into a non-pulsed flow free from exhaust interference.
 2. The heat exchanger according to claim 1, wherein the first heat exchanger (H2 to H4) positioned on an upstream side of the flow of the exhaust gas and the second heat exchanger (H1) positioned on a downstream side communicate with each other.
 3. The heat exchanger according to claim 1, wherein the first heat exchanger (H2 to H4) and the second heat exchanger (H1) are integrated.
 4. The heat exchanger according to claim 1, wherein the second heat exchanger (H1) is disposed in a layer around an outer periphery of the first heat exchanger (H2 to H4).
 5. The heat exchanger according to claim 1, wherein the flow of the exhaust gas exiting from the first heat exchanger (H2 to H4) changes direction by 1800 before entering the second heat exchanger (H1).
 6. The heat exchanger according to claim 1, wherein a flow of exhaust gas is from front-to-back in one portion of the first heat exchanger (H2 to H4) and is from back-to-front in another portion of the first heat exchanger (H2 to H4), the one portion and the another portion being separate from each other.
 7. A heat exchanger for a multicylinder internal combustion engine, comprising: a first heat exchanger (H2 to H4) disposed in each exhaust passage (33) extending from a corresponding combustion chamber (16) of the multicylinder internal combustion engine (1) toward an exhaust pipe (32) at a location downstream from an exhaust valve (25), and a second heat exchanger (H1) disposed in a section where at least two exhaust passages (33) are combined, wherein a heat medium flows from the second heat exchanger (H1) to the first heat exchanger (H2 to H4).
 8. The heat exchanger for a multicylinder internal combustion engine according to claim 7, wherein the first heat exchanger (H2 to H4) positioned on an upstream side of a flow of exhaust gas and the second heat exchanger (H1) positioned on a downstream side communicate with each other.
 9. The heat exchanger for a multicylinder internal combustion engine according to claim 7, wherein the first heat exchanger (H2 to H4) and the second heat exchanger (H1) are integrated.
 10. The heat exchanger for a multicylinder internal combustion engine according to claim 7, wherein the second heat exchanger (H1) is disposed in a layer around an outer periphery of the first heat exchanger (H2 to H4).
 11. The heat exchanger for a multicylinder internal combustion engine according to claim 7, wherein the first heat exchanger is a plurality of first exchangers each one being disposed inside one of said exhaust passages, and the second heat exchanger extends across said section where at least two of said exhaust passages are combined.
 12. A heat exchanger for a multicylinder internal combustion engine having a plurality of cylinders and exhaust passages extending from respective cylinders, comprising: a first heat exchanger (H2 to H4) disposed in each of said exhaust passages (33) extending from a corresponding combustion chamber (16) of the multicylinder internal combustion engine (1), and a second heat exchanger (H1) disposed in a section where at least two of the exhaust passages (33) are combined, wherein at least a part of said first heat exchanger (H2 to H4) is housed inside a cylinder head (12) of the engine such that said at least two exhaust passages communicate with said section and exhaust gases from the respective combustion chambers flow through said at least two exhaust passages, respectively, and then through said section, wherein at said section, the exhaust gases are combined into a non-pulsed flow free from exhaust interference.
 13. The heat exchanger according to claim 12, wherein the first heat exchanger (H2 to H4) positioned on an upstream side of a flow of exhaust gas and the second heat exchanger (H1) positioned on a downstream side communicate with each other.
 14. The heat exchanger according to claim 12, wherein the first heat exchanger (H2 to H4) and the second heat exchanger (H1) are integrated.
 15. The heat exchanger according to claim 12, wherein the second heat exchanger (H1) is disposed in a layer around an outer periphery of the first heat exchanger (H2 to H4).
 16. The heat according to claim 12, wherein the first heat exchanger is a plurality of first exchangers each one being disposed inside one of said exhaust passages, and the second heat exchanger extends across said section where at least two of said exhaust passages are combined.
 17. The heat exchanger according to claim 12, wherein a flow of exhaust gas is from front-to-back in one portion of the first heat exchanger (H2 to H4) and is from back-to-front in another portion of the first heat exchanger (H2 to H4), the one portion and the another portion being separate from each other.
 18. A heat exchanger for a multicylinder internal combustion engine, comprising: a first heat exchanger (H2 to H4) disposed in each exhaust passage (33) extending from a corresponding combustion chamber (16) of the multicylinder internal combustion engine (1) toward an exhaust pipe (32) at a location downstream from an exhaust valve (25), and a second heat exchanger (H1) disposed in a section where at least two of the exhaust passages (33) are combined, wherein at least a part of said first heat exchanger (H2 to H4) is housed inside a cylinder head (12) of the engine, and wherein a heat medium flows from the second heat exchanger (H1) to the first heat exchanger (H2 to H4).
 19. The heat exchanger for a multicylinder internal combustion engine according to claim 18, wherein the first heat exchanger (H2 to H4) positioned on an upstream side of a flow of exhaust gas and the second heat exchanger (H1) positioned on a downstream side communicate with each other.
 20. The heat exchanger for a multicylinder internal combustion engine according to claim 18, wherein the first heat exchanger (H2 to H4) and the second heat exchanger (H1) are integrated.
 21. The heat exchanger for a multicylinder internal combustion engine according to claim 18, wherein the second heat exchanger (H1) is disposed in a layer around an outer periphery of the first heat exchanger (H2 to H4).
 22. The heat exchanger for a multicylinder internal combustion engine according to claim 18, wherein the first heat exchanger is a plurality of first exchangers each one being disposed inside one of said exhaust passages, and the second heat exchanger extends across said section where at least two of said exhaust passages are combined. 